The present invention relates to offshore drilling apparatus and more particularly to fluid flow measurement apparatus and surge and ebb compensators to be employed therewith.
In most well drilling operations a supply of so-called "drilling mud" is circulated passed the drilling head during the actual drilling operation. The composition of the mud flow rates employed are critical parameters in the drilling operation. Accordingly, it is common practice to incorporate a flow meter in the line wherein the drilling mud is flowing to measure the flow rate of the mud therethrough.
When the drilling operation is at an offshore location, the severity of the environment adds a number of additional consideration into all aspects of the drilling operation. In such operations, a riser pipe extends from the ocean floor where the drill head is located to a floating platform or ship on the surface having the pumping apparatus and the like. The piping carried by the floating platform and the riser pipe attached adjacent the ocean floor are interconnected by a telescoping section which provides for a variable length conduit between the fixed and floating portions. With a "constant" flow of mud through the riser pipe, the flow is subjected to surges and ebbs as the platform rises and falls with the relative position of the water causing the telescoping section to extend and contract respectively. As the telescoping section extends, the total internal volume of the conduit between the ocean floor and the floating platform increases, causing an ebb in the flow of the mud during the transition period. As the platform falls, the telescoping section contracts and the internal volume decreases, causing a temporary surge in the mud flow rate. Such ebbs and surges in the flow rate make the monitoring of the flow rate a difficult task subject to inaccuracies.
Both mechanical and electrical compensators have been proposed as solutions to the problem. Electronic compensators, of course, only approximate or anticipate what compensation factors should be applied. Likewise, electronic compensators are much more delicate and are prone to damage within the rough-and-tumble environment of an offshore drilling rig. A mechanical compensator on the other hand, has inherent qualities of great durability and, as will be seen, by being incorporated directly within the operating fluid flow system, operates directly in combination therewith to compensate for conditions as they actually exist.
One form of mechanical compensator recently developed is shown in FIGS. 1 and 2. The riser pipe 10 extends from the well head (not shown) on the ocean floor to connect to a pipe 12 carried by a floating ship or platform as characterized by the I-beam 14. Riser pipe 10 and pipe 12 are interconnected by a telescoping section 16 which can extend and contract along its longitudinal axis to compensate for changes in length between the ocean floor and the floating platform as the floating platform rises and falls on the ocean's surface. As shown in FIG. 1, the telescoping section 16 is in a substantially extended condition corresponding to the ship rising on the ocean's surface. In FIG. 2, the telescoping section 16 is in a substantially fully contracted position corresponding to a position of the floating platform at maximum fall with respect to the ocean's floor. A process pipe 18 is connected to pipe 12 to conduct the drilling mud in its flow. It is the flow through process pipe 18 which must be monitored for flow rate. It is also the flow-through process pipe 18 which is subjected to ebbs as the telescoping section 16 moves from the position of FIG. 2 towards the position of FIG. 1 and surges as the telescoping section 16 moves from the position of FIG. 1 towards the position of FIG. 2. In the prior art compensating apparatus of FIG. 1 and FIG. 2, a compensating telescoping section generally indicated as 20 is provided. The internal cross-sectional area of compensating telescoping section 20 by necessity, is made identical to that of telescoping section 16. Compensating telescoping section 20 comprises a first or upper portion 22 which is closed on the outer end and is rigidly carried by the fixed portion of telescoping section 16 connected to riser pipe 10. Compensating telescoping section 20 also has a second or sliding portion 24 which is movable longitudinally along the first portion 22. Second portion 24 is connected by a flexible conduit 26 to riser pipe 10. Additionally, second portion 24 is connected (as with cables 28) to the floating platform as represented by I-beam 14. Thus, as the platform rises and falls with relation to the ocean floor, second portion 24 will move in combination therewith. As a result, as telescoping section 16 is extended any distance, compensating telescoping section 20 will be contracted an equal amount. Since the cross-sectional areas of the two are identical, the changes in internal volume of the telescoping sections 16 and 20 will be equal and opposite. Thus, in the extension of telescoping section 16 shown in FIG. 1, additional mud from within telescoping section 20 will flow from flexible conduit 26 into riser pipe 10 (as indicated by the arrow 30) to offset the tendency for an ebb in the flow of the mud therein. In similar manner, in the contraction of telescoping section 16 shown in FIG. 2 as telescoping section 16 contracts, compensating telescoping section 20 will extend causing the potential surge of mud within riser pipe 10 to flow through flexible conduit 26 (as indicated by the arrow 30') into the increasing internal volume of compensating telescoping section 20 to absorb the additional mud flow and, thereby, eliminate the surge within process pipe 18.
As can be seen, the mechanical prior art compensating technique shown in FIG. 1 and FIG. 2 provides workable method for precisely compensating for ebbs and surges within riser pipe 10 caused by the extension and contraction of the telescoping section 16 thereof. As can be surmised from the construction shown in FIG. 1 and FIG. 2, however, the aforementioned prior art technique also poses certain undesirable constraints. For example, since first portion 22 is carried by riser pipe 10 or the fixed portion of telescoping section 16 attached thereto, the two portions must be placed close adjacent one another. This has numerous drawbacks. First, telescoping section 16 may be disposed in an inaccessible spot with relation to the platform or ship. Second, once the compensating telescoping section 20 is operably connected to perform its compensating function, it cannot be quickly and easily removed. Third, it is not easy to install such apparatus on previously installed drilling apparatus or to remove it once it has been installed. Fourth, since the telescoping sections move longitudinally in unison, the internal cross-sectional areas of the two telescoping sections must be identical. Fifth, such apparatus is not easily turned "off" and "on" with respect to accomplishing its compensating function. Last, any flexible connector is a weak link subject to breakage.
Wherefore, it is the object of the present invention to provide a mechanical compensating apparatus for use in combination with undersea drilling operations or similar applications which can be located remote from the telescoping section of the riser pipe, can be easily installed on existing drilling rigs, can be disconnected and reconnected easily at will, is structurally sound, and is not constrained to incorporating a telescoping section substantially identical to that connected in the riser pipe.